Journal of Oceanography

, Volume 74, Issue 2, pp 169–186 | Cite as

A three-dimensional numerical study of river plume mixing processes in Otsuchi Bay, Japan

  • Kaushik Sasmal
  • Eiji Masunaga
  • Adrean Webb
  • Oliver B. Fringer
  • Edward S. Gross
  • Matthew D. Rayson
  • Hidekatsu Yamazaki
Original Article


The three-dimensional numerical model SUNTANS is applied to investigate river plume mixing in Otsuchi Bay, an estuary located along the Sanriku Coast of Iwate, Japan. Results from numerical simulations with different idealized forcing scenarios (barotropic tide, baroclinic tide, and diurnal wind) are compared with field observations to diagnose dominant mixing mechanisms. Under the influence of combined barotropic, baroclinic and wind forcing, the model reproduces observed salinity profiles well and achieves a skill score of 0.94. In addition, the model forced by baroclinic internal tides reproduces observed cold-water intrusions in the bay, and barotropic tidal forcing reproduces observed salt wedge dynamics near the river mouths. Near these river mouths, vertically sheared flows are generated due to the interaction of river discharge and tidal elevations. River plume mixing is quantified using vertical salt flux and reveals that mixing near the vicinity of the river mouth, is primarily generated by the barotropic tidal forcing. A 10 ms−1 strong diurnal breeze compared to a 5 ms−1 weak breeze generates higher mixing in the bay. In contrast to the barotropic forcing, internal tidal (baroclinic) effects are the dominant mixing mechanisms away from the river mouths, particularly in the middle of the bay, where a narrow channel strengthens the flow speed. The mixing structure is horizontally asymmetric, with the middle and northern parts exhibiting stronger mixing than the southern part of the bay. This study identifies several mixing hot-spots within the bay and is of great importance for the coastal aquaculture system.


River plume mixing Estuary Barotropic tide Baroclinic tide Diurnal breeze Salt wedge Cold-water intrusion SUNTANS model 



This study was supported by a grant from Tohoku Ecosystem Associated Marine Science (TEAMS), a research program launched by the Ministry of Education, Culture, Sports, Science and Technology (MEXT). The authors wish to express sincere appreciation to the Captains of M. Kurosawa and M. Hirano for their dedicated work in our field surveys. The authors are also grateful to Rusty Holleman who designed the grid generation tool that was used to create the computational domain for this study.


  1. Allen JI, Somerfield PJ, Gilbert FJ (2007) Quantifying uncertainty in high-resolution coupled hydrodynamic-ecosystem models. J Mar Syst 64(1):3–14CrossRefGoogle Scholar
  2. Anbo A, Otobe H, Takagi M (2005) On the river water dis- charged into Otsuchi Bay. Rep Int Coast Mar Res Center 30:4–8 (in Japanese) Google Scholar
  3. Banas NS, Hickey BM (2005) Mapping exchange and residence time in a model of Willapa Bay, Washington, a branching, macrotidal estuary. J Geophys Res 110:C11011. doi: 10.1029/2005JC002950 CrossRefGoogle Scholar
  4. Chua VP, Fringer OB (2011) Sensitivity analysis of three-dimensional salinity simulations in North San Francisco Bay using the unstructured-grid SUNTANS model. Ocean Model 39(3):332–350CrossRefGoogle Scholar
  5. Fong DA, Geyer WR (2001) Response of a river plume during an upwelling favorable wind event. J Geophys Res 106(C1):1067–1084CrossRefGoogle Scholar
  6. Fringer OB, Gerritsen M, Street RL (2006) An unstructured-grid, finite-volume, nonhydrostatic, parallel coastal ocean simulator. Ocean Model 14(3):139–173CrossRefGoogle Scholar
  7. Furuya K, Takahashi K, Iizumi H (1993) Wind-dependent formation of phytoplankton spring bloom in Otsuchi Bay, a ria in Sanriku, Japan. J Oceanogr 49(4):459–475CrossRefGoogle Scholar
  8. Galperin B, Kantha LH, Hassid S, Rosati A (1988) A quasi-equilibrium turbulent energy model for geophysical flows. J Atmos Sci 45(1):55–62CrossRefGoogle Scholar
  9. Garrett C, Gilbert D (1988) Estimates of vertical mixing by internal waves reflected off a sloping bottom. Elsevier Oceanogr Ser 46:405–423CrossRefGoogle Scholar
  10. Geyer WR (1997) Influence of wind on dynamics and flushing of shallow estuaries. Estuar Coast Shelf Sci 44:713–722CrossRefGoogle Scholar
  11. Geyer WR, MacCready P (2014) The estuarine circulation. Annu Rev Fluid Mech 46(1):175–197CrossRefGoogle Scholar
  12. Gill AE (1982) Atmosphere–ocean dynamics. Academic PressGoogle Scholar
  13. Gille ST, Llewellyn Smith SG, Lee SM (2003) Measuring the sea breeze from QuikSCAT scatterometry. Geophys Res Lett 30:1114. doi: 10.1029/2002GL0162303 CrossRefGoogle Scholar
  14. Gille ST, Llewellyn Smith SG, Statom NM (2005) Global observations of the land breeze. Geophys Res Lett 32:L05605. doi: 10.1029/2004GL022139 CrossRefGoogle Scholar
  15. Hetland RD (2005) Relating river plume structure to vertical mixing. J Phys Oceanogr 35(9):1667–1688CrossRefGoogle Scholar
  16. Holleman R, Fringer O, Stacey M (2013) Numerical diffusion for flow-aligned unstructured grids with application to estuarine modeling. Int J Numer Methods Fluids 72(11):1117–1145CrossRefGoogle Scholar
  17. Horner-Devine AR, Hetland RD, MacDonald DG (2015) Mixing and transport in coastal river plumes. Annu Rev Fluid Mech 47:569–594CrossRefGoogle Scholar
  18. Hunter E, Chant R, Bowers L, Glenn S, Kohut J (2007) Spatial and temporal variability of diurnal wind forcing in the coastal ocean. Geophys Res Lett 34:L03607. doi: 10.1029/2006GL028945 CrossRefGoogle Scholar
  19. Jurisa JT, Chant RJ (2013) Impact of offshore winds on a buoyant river plume system. J Phys Oceanogr 43(12):2571–2587CrossRefGoogle Scholar
  20. Kishi MJ, Higashi N, Takagi M, Sekiguchi K, Otobe H, Furuya K, Aiki T (2003) Effect of aquaculture on material cycles in Otsuchi Bay, Japan. Otsuchi Mar Sci 28:65–71Google Scholar
  21. Ledwell JR, Montgomery ET, Polzin KL, Laurent LS, Schmitt RW, Toole JM (2000) Evidence for enhanced mixing over rough topography in the abyssal ocean. Nature 403(6766):179–182CrossRefGoogle Scholar
  22. MacCready P, Banas NS, Hickey BM, Dever EP, Liu Y (2009) A model study of tide-and wind-induced mixing in the Columbia River Estuary and plume. Cont Shelf Res 29(1):278–291CrossRefGoogle Scholar
  23. MacDonald DG, Geyer WR (2004) Turbulent energy production and entrainment at a highly stratified estuarine front. J Geophys Res 109:C05004. doi: 10.1029/2003JC002094 CrossRefGoogle Scholar
  24. Masunaga E, Yamazaki H (2014) A new tow-yo instrument to observe high-resolution coastal phenomena. J Mar Syst 129:425–436CrossRefGoogle Scholar
  25. Masunaga E, Homma H, Yamazaki H, Fringer OB, Nagai T, Kitade Y, Okayasu A (2015) Mixing and sediment resuspension associated with internal bores in a shallow bay. Cont Shelf Res 110:85–99CrossRefGoogle Scholar
  26. Masunaga E, Fringer OB, Yamazaki H (2016a) An observational and numerical study of river plume dynamics in Otsuchi Bay. J Oceanogr, Japan. doi: 10.1007/s10872-015-0324-2 Google Scholar
  27. Masunaga E, Fringer OB, Yamazaki H, Amakasu K (2016b) Strong turbulent mixing induced by internal bores interacting with internal tide-driven vertically-sheared flow. Geophys Res Lett. doi: 10.1002/2016GL067812 Google Scholar
  28. Mellor GL, Yamada T (1982) Development of a turbulence closure model for geophysical fluid problems. Rev Geophys 20(4):851–875CrossRefGoogle Scholar
  29. Millero FJ, Chen CT, Bradshaw A, Schleicher K (1980) A new high pressure equation of state for seawater. Deep Sea Res Part A Oceanogr Res Papers 27(3):255–264CrossRefGoogle Scholar
  30. Murphy AH (1988) Skill scores based on the mean square error and their relationships to the correlation coefficient. Mon Weather Rev 116(12):2417–2424CrossRefGoogle Scholar
  31. Nakatsuka T, Toda M, Kawamura K, Wakatsuchi M (2004) Dissolved and particulate organic carbon in the Sea of Okhotsk: transport from continental shelf to ocean interior. J Geophys Res. doi: 10.1029/2003JC001909 Google Scholar
  32. Nash JD, Moum JN (2005) River plumes as a source of large-amplitude internal waves in the coastal ocean. Nature 437(7057):400–403CrossRefGoogle Scholar
  33. Okazaki M (1990) Internal tidal waves and internal long period waves in the Sanriku coastal seas, eastern coast of northern Japan. La mer 28:5–29Google Scholar
  34. Orton PM, McGillis WR, Zappa CJ (2010) Sea breeze forcing of estuary turbulence and air-water CO2 exchange. Geophys Res Lett 37:L13603. doi: 10.1029/2010GL043159 CrossRefGoogle Scholar
  35. Otobe H, Onishi H, Inada M, Michida Y, Terazaki M (2009) Estimation of water circulation in Otsuchi Bay, Japan inferred from ADCP observation. Coast Mar Sci 33(1):1–9Google Scholar
  36. Pietrzak JD, Labeur RJ (2004) Trapped internal waves over undular topography in a partially mixed estuary. Ocean Dyn 54(3–4):315–323Google Scholar
  37. Pineda J (1994) Internal tidal bores in the nearshore: warm-water fronts, seaward gravity currents and the onshore transport of neustonic larvae. J Mar Res 52(3):427–458CrossRefGoogle Scholar
  38. Pineda J (1995) An internal tidal bore regime at nearshore stations along western USA: predictable upwelling within the lunar cycle. Cont Shelf Res 15(8):1023–1041CrossRefGoogle Scholar
  39. Pond S, Pickard GL (1983) Introductory dynamical oceanography, 2nd edn. Pergamon Press, Tarrytown, p 329Google Scholar
  40. Ralston DK, Geyer WR, Lerczak JA, Scully M (2010) Turbulent mixing in a strongly forced salt wedge estuary. J Geophys Res 115:C12024. doi: 10.1029/2009JC006061 CrossRefGoogle Scholar
  41. Rayson MD, Gross ES, Fringer OB (2015) Modeling the tidal and sub-tidal hydrodynamics in a shallow, micro-tidal estuary. Ocean Model 89:29–44CrossRefGoogle Scholar
  42. Richards C, Bourgault D, Galbraith PS, Hay A, Kelley DE (2013) Measurements of shoaling internal waves and turbulence in an estuary. J Geophys Res 118(1):273–286CrossRefGoogle Scholar
  43. Shikama N (1980) Current measurements in Otsuchi Bay. Bull Coast Oceanogr 18:1–8Google Scholar
  44. Shikama N (1986) Exchange mechanisms of water in a ria-type bay along Sanriku coast. Bull Jpn Soc Fish Oceanogr 50:170–174Google Scholar
  45. Shikama N (1990) Characteristics in flow field of water in Otsuchi Bay. Otsuchi Mar Res Centre Rep 16:75 (in Japanese) Google Scholar
  46. Tanaka K, Komatsu K, Itoh S, Yanagimoto D, Ishizu M, Hasumi H, Michida Y (2015) Baroclinic circulation and its high frequency variability in Otsuchi Bay on the Sanriku ria coast. J Oceanogr, Japan. doi: 10.1007/s10872-015-0338-9 Google Scholar
  47. Valle-Levinson A (ed) (2010) Contemporary issues in estuarine physics. Cambridge University Press, CambridgeGoogle Scholar
  48. van Haren H (2009) Using high sampling-rate ADCP for observing vigorous processes above sloping (deep) ocean bottoms. J Mar Syst 77(4):418–427CrossRefGoogle Scholar
  49. Walsh JJ (1991) Importance of continental margins in the marine biogeochemical cycling of carbon and nitrogen. Nature 350(6313):53–55CrossRefGoogle Scholar
  50. Walter RK, Woodson CB, Arthur RS, Fringer OB, Monismith SG (2012) Nearshore internal bores and turbulent mixing in southern Monterey Bay. J Geophys Res 117(C07017):1–13Google Scholar
  51. Wang B, Fringer OB, Giddings SN, Fong DA (2009) High-resolution simulations of a macrotidal estuary using SUNTANS. Ocean Model 28(1):167–192CrossRefGoogle Scholar
  52. Wang B, Giddings SN, Fringer OB, Gross ES, Fong DA, Monismith SG (2011) Modeling and understanding turbulent mixing in a macrotidal salt wedge estuary. J Geophys Res 116:1–23Google Scholar
  53. Xing J, Davies AM (1999) The effect of wind direction and mixing upon the spreading of a buoyant plume in a non-tidal regime. Cont Shelf Res 19(11):1437–1483CrossRefGoogle Scholar

Copyright information

© The Oceanographic Society of Japan and Springer Japan KK 2017

Authors and Affiliations

  • Kaushik Sasmal
    • 1
  • Eiji Masunaga
    • 2
  • Adrean Webb
    • 3
  • Oliver B. Fringer
    • 4
  • Edward S. Gross
    • 5
  • Matthew D. Rayson
    • 6
  • Hidekatsu Yamazaki
    • 7
  1. 1.Department of Ocean Technology, Policy, and Environment, Graduate School of Frontier SciencesThe University of TokyoKashiwaJapan
  2. 2.Centre for Water Environment StudiesIbaraki UniversityItakoJapan
  3. 3.Disaster Prevention Research InstituteKyoto UniversityKyotoJapan
  4. 4.Department of Civil and Environmental EngineeringStanford UniversityStanfordUSA
  5. 5.Center for Watershed SciencesUniversity of CaliforniaDavisUSA
  6. 6.School of Civil, Environmental and Mining Engineering and the Oceans InstituteUniversity of Western AustraliaCrawleyAustralia
  7. 7.Department of Ocean SciencesTokyo University of Marine Science and TechnologyTokyoJapan

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